[FastISel][AArch64] Try to fold the offset into the add instruction when simplifying...

[FastISel][AArch64] Try to fold the offset into the add instruction when simplifying a memory address.

Small optimization in 'simplifyAddress'. When the offset cannot be encoded in
the load/store instruction, then we need to materialize the address manually.
The add instruction can encode a wider range of immediates than the load/store
instructions. This change tries to fold the offset into the add instruction
first before materializing the offset in a register.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218031 91177308-0d34-0410-b5e6-96231b3b80d8